High-pressure metal vapor electric discharge lamp



Oct. 3l, 1939.- H. G. JENKINS Erm. 2,177,712

HIGH-PRESSURE METAL VAPOR ELECTRIC DISCHARGE LAMP Filed Nov. 10, 1937 2Sheets-Sheet l Figl TC 360 George H. Wilso BYW AT ORNEY O 2O ,40 60 BO|00 |20 v|40 LGO |80 200 220 240 ZGQ Oct. 31, 1939. H. G. JENKINS Erm.

HIGH-PRES'SUREMETAL VAPOR ELECTRIC DISCHARGE LAMP Filed Nov. 1o, 1937 C2 Sheets-Sheet 2 c zoo |60 |60 |40 |20 loo Fig 3 XII/ao escen' l fCoating /3 INVENTORS C0 Henrg G. Jenkn John W. Ryde George H. BY

Wan/zy ATTORNEY Patentedv Oct. 31', `19309 DISCHARGE Henry GraingerJenkins, Pinner, John Waiter Ryde, London, and George Hulbert Wilson,Wembley Park, Middlesex, England, assignors to General Electric Company,a corporation of New York ik Application Nnvmber 1o, 1931, sensi No.113,182 I n Great Britain November 11, 1936 1 Cla-im.A (Ci. 176-.-122)This invention relates -to electric discharge lamps of the typecomprising an envelope `through which a high-pressure metal-vapor (HPMV)dlschargeis adapted to passandmas terial. (hereinafter calledluminescent material)v outside that envelope adapted` to be excitedtoluminescence by `radiation from that discharge.

Thefunction of the luminescent material is usually to correct the colourof the discharge, that is to say, to supply visible radiation ofwavelengths in `which the radiation of the discharge is deiiclent; thusthe lamp `may beof the kind described and claimedy in British patentspecification No. 401,846. On the other hand the'function of theluminescent material maybe to increase the total `luminous outputwithout regard to colour. In either casethe envelope must, of course,transmit .radiation capablevfiof exciting the luminescent material;vButnthis does not mean that the envelope. mustfbeof quartz, at any ratevif thepurpose ofthe luminescent material is colour correction.- lFor,asindicated in the said` specification, zinc-cadmium sulphide activatedby copper, which is` excitedmby radiaf tion to which glass istransparent. is a suitable material for supplying red light toIsupplement the deilciency of the mercury spectrum in that colour.

The most convenient place'ifor the luminescent material ishon theinsiderpf the'4 outerv jacket. But the` temperature oi'.this jacket,though below .that of lthe inner envelopais oitenmuch above normalatmospheric temperature. Figure, 1 of the accompanying drawings :showsthe ,temperaf ture-distribution over the `outer -jacket of a.400- watthigh, intensity` mercury vapor lamp of a well-known commercial, type,much. used `for` street-lighting and like purposes.A ,Here line i is ahalf-sectionfof the inner envelope, mm.Y

'. in diameter, containing the velectrodesfl and 5,

outer jacket, mm. in diameter. Line 3referred' to vaxes X, T,- gives the`temperature T at oni the outer .jacket distant. X. from one end oi' theouterjacket. It will kbe seen that lthe temperature is considerably morethan 300 C. over a considerable central portion,

Now fewlu minescent materials operate satisfactorily at such atemperature. Their eiilciency 56 usually decreases notably `even at atemperature as low as 180" C. The decrease is partly reversible andisrestored when the temperature is reduced. but it is also partlypermanent if -the high temperature, combined with strong illumlnation,is maintained for many hours. The

160 mm.' apart, Vline 2 isla half-section of the lreversible change ismainlyxvor entirely determined by the nature of the material itself thepermanent change depends largely on the gases to which the material issubjected. But it is unnecessary to distinguish between. permanent 5 andreversible changes, since ,the temperatures at which they have to betaken into account are not lvery widely diierent; for either of them, incircumstances with whichthis invention is con;- cerned, thelcriticaltemperature that must not be exceeded (which'of lcoursedependson the eiliciency and on the permanence required) usul ally, if notalways, lies between `140 C. and 300 C.

One method of avoiding lossof eiliciency` due to high temperature is to`put the luminescent .'material, not in thelamp itself, but on somepaitv of a tting associated with the lamp, but sunlciently far from itto beatsubstantially atmoli-l phericvtemperature. But then it is notalways g easy to secure that the primary light` from the 1 discharge andthe secondary lightfrom the luyminescent material are adequately mixed,asis often required. It would be more'convenient l if the materialcouldstill 'beplacedon the outer jacket, which must then benso largeythat it is. below the critical temperature.` 1 i;

- If the energydissipated-in the lampis sumciently small, for example-.wattsfor less, and

lmore particularly if, the;` length ofthe discharge a `column isl small,for 4exampleless than20mm1w.

(these conditions areMfullled/in knowmlampsfV -I yoperating Vatvaporpressuresmuch exceedin'gh` one atmosphere) ,the` size of s.spherical; jorpos-r, siblycylindrical, outer jacket whose tempera'`ture-is nowhere above-'? C. is notfseriouslyf inconvenient.' `Butwhenfthe length ofzthe discharge Aexceeds 100mm. and thewattsdissipated'- veirceed1250A the lsizexii a spherical or cylindricall fjacket yfulfilling this condition becomes incon-` venient. `Theobject of -this invention is to avoid" l ythis inconvenience and toprovide jackets ful--f` ,flling this condition `that arentincon'veniently large. l

When the maximum temperature Tui of the jacket is given, 4the smallestpossible jacket, if

would distort these currents; it appears improbable that anyjacket-cooled by convection could have a uniform temperature all overits surface.

In these circumstances, the smallest jacket will probably be the mostnearly isothermal, that is to say, that which for a given Tm has thehighest mean temperature. It is probable further that there is only onesuch surface (other conditions' being constant, such as the material andthe thickness of the jacket and the medium between it and the dischargeenvelope) or at most only a few such surfaces. The question thereforearises whether such a substantially isothermal jacket will bepracticable on other grounds, for example whether it will be sui-`ciently small and of a shape that can easily be made and coated withluminescent materials and that will lead to an adequate mixing of theprimary and secondary light. The question can be answered only byexperiment'.

The discovery on which this invention is based is that the answer isaffirmative. When the lamp is operated with the discharge columnvertical, a jacket of a simple pear-shape, with the narrow end downwardsapproximates closely to the most nearly isothermal surface, is notunduly bulky, and is otherwise suitable.

In applying this discovery, it must be remembered that, if the dischargecolumn is vertical and the space between the inner envelope and outerjacket is filled with gas at a pressure so low that convection in ittransfers little heat to the jacket, the coolest parts of the jacketwill always be at the upper and lower ends, towards which littleradiation is emitted. In order to bring these ends up to nearly themaximum temperature in the central portion of the jacket, the ends wouldprobably have to be re-entrant. This would be a disadvantage, not anadvantage; accordingly in estimating whether a sha`pe approximatesclosely to the most nearly isothermal surface, for the purpose of theinvention, the ends through which not more than 10% of the totalradiation issues may be neglected; a shape is not to be regarded as notapproximately substantially isothermal, merely because, by a re-shapingof these ends, it could be made more nearly isothermal. 'I'he truecriterion of approach to the most nearly isothermal surface is theuniformity of temperature over that part of the surface through whichthe greater part of the radiation issues.

To determine how far any given shape approximates to the most nearlyisothermal, considerable experimenting is required. Accordingly wepropose to define our invention, not in terms of the most nearlyisothermal surface, but in terms of the temperature distribution overthe jackets that our experiments have shown to approach as nearly aspossible (subject to the foregoing consideration) to that surface. Theseexperiments show that it is possible to obtain jackets in which, overthe whole of a surface on which '75% of the total radiation falls thetemperature does not vary by as much as 15 C. and in which over thewhole ofa surface on which 90% of the total radiation falls thetemperature does not vary more than 50 C.

According to the invention in a lamp of the type specified wherein thedischarge is adapted to consume more than 250 watts and in which thedischarge column `is more than 100 mm. long, the luminescent material iscarried on the interior surface of a sealed outer jacket surrounding thedischarge envelope, the space between the said jacket and envelope beingoccupied by a gas at a pressure greatly below atmospheric, and the saidjacket is so shaped that, when exposed in free air in full operation,the temperature over a part of the surface of the said jacket on which75% of the radiation falls does not vary by as much as 15 C., the meantemperature of this part of the surface lying between 140 C. and 300 C.,and the temperature over a part of the surface on which 90% of theradiation falls does not Vary by more than 50 C.

One embodiment of the invention will now be described with reference toFig. 3 of the accompanying drawings, which show a broken -axial sectionthrough a lamp and a curve indicating the distribution of temperatureover it.

In the drawings II is the glass inner envelope of a 400 watthigh-pressure mercury-vapor discharge lamp of a known commercial type.It hangs vertically, as usual, from the screw cap I2. From this caphangs also the sealed pear-shaped outer jacket I3, which is of glassabout 1 mm. thick, coated internally with zinc-cadmium sulphideactivated by copper. The parts are drawn to scale, the dimensions markedA, B, C being respectively 340, 165, 52 mm. The space between the jacketII and the vessel I3 is filled with dry oxygen at 10 mm. pressure. Thejacket I3 is surrounded by free air, so that the temperature issubstantially constant along any circle on its surface in a horizontalplane.

The chart alongside the fig-ure shows in a manner obvious to thoseskilled in the art how this temperature varies over the surface. Theternperature is given only for the region between the horizontal planesX and Y. The proportion of the radiation falling on the jacket outsidethese planes is considerably less than 10%; the planes could be movedtowards each other so that each passed through the adjacent points at atemperature of 130 C. and the proportion of the radiation fallingoutside them would still be less than 10%. Accordingly in thisembodiment the temperature over a part of the surface on which 90% ofthe radiation falls does not vary by more than 40%. 'I'he central partof the surface over which the temperature does not vary by 15 C.receives at least 75% of the radiation and the mean temperature of thispart is between 150 and 160 C.

In order to ascertain how far these measurements could berepeated, twoother lamps intended to be similar were made and the temperatures overtheir surfaces measured. The results are shown in Figure 2 of theaccompanying drawings in a manner similar to that adopted in Fig. 3. Thetemperature measurements extend from the bottom of the jacket (X=) to apoint on the neck of the jacket 250 mm. distant. The points indicated bycrosses refer to tests on one of these lamps, those indicated by'circlesrefer to the other; the dotted line is the temperature curve of Fig. 3reproduced for comparison.

It is to be observed that there are minor discrepancies which are, inpart at least, due to experimental errors. If the average error for alarge number of lamps was drawn, apparent irregularities would besmoothed out and the said limit of 15 C. could certainly be reduced,probably to C.

It is not necessary that the Whole of the interior of the jacket shouldbe coated with the luminescent material; but no advantage is known fromleaving any considerable part of it uncovered,

except possibly if it is desired to obtain lights of aviaria differentcolour in different directions. The luminescent material on the jacketwill act to some extent as a diffusing surface, promoting the mixture ofprimary and secondary light. But the surface of the globe, either thatwhich bears the luminescent material or that which does not, or theglass of which it is composed, may be rendered diffusing in known mannerso as to promote mixture further. Again the lamp may be associated withdiffusing surfaces outside the globe, so long as they form part of afitting in which the lamp is placed and not an integral part of it, soas to increase its bulk. But it is not always necessary to produce aperfect mixture of the primary and secondary light; that is to say, theratio of the quantity of secondary to the quantity` of primary lightreceived by an observer at a distance may vary with the position of theobserver. 'I'hus in street lighting it is generally permissible that thelight emitted at large angles to the vertical should be less perfectlycorrected for colour than light emitted near the vertical; such adifference is likely to bev secured by the use of a. jacket according tothe invention, without subsidiary diiusing means.

We claim:

An electric gaseous discharge lamp comprising an elongated tubular vaporarc device of the type adapted to operate with a constricted arcdischarge, a jacket about said device, said jacket having a modiedprolatev spheroidal shape wherein the lower end is more drawn out thanthe upper, the distance of every point on said jacket from said device,with the exception of those portions lying beyond the ends of saiddevice, being A such that when said lamp is operated with the axis ofsaid device vertical the temperature of said points does not varysubstantially from the mean temperature thereof, and a coating of auorescent material on the inner surface of said jacket.

HENRY GRAINGER JENKINS.

' JOHN WALTER RYDE.

GEORGE HULBERT WILSON.

